Processes for producing metal parts from ferrous powders using powder metallurgy (P/M) techniques are well known. Such techniques typically involve mixing of ferrous powders with alloying components such as graphite, copper or nickel in powder form, filling the die with the powder mixture, compacting and shaping of the compact by the application of pressure, and ejecting the compact from the die. The compact is then sintered wherein metallurgical bonds are developed by mass transfer under the influence of heat. The presence of an alloying element enhances the strength and other mechanical properties in the sintered part compared to the ferrous powders alone. When necessary, secondary operations such as sizing, coining, repressing, impregnation, infiltration, machining, joining, etc. are performed on the P/M part.
It is common practice to use a lubricant for the compaction of ferrous powders. It is required mainly to reduce the friction between metal powders and die walls. By ensuring a good transfer of the compacting force during the compaction stage, it improves the uniformity of densification throughout the part. Besides, it also lowers the force required to remove the compact from the die, thus minimizing die wear and yielding parts with good surface finish.
The lubricant can be admixed with the ferrous powders or sprayed onto the die walls before the compaction. Die-wall lubrication is known to give rise to compacts with high green strength. Indeed, die-wall lubrication enables mechanical anchoring and metallurgical bonding between particles during compaction. However, die-wall lubrication increases the compaction cycle time, leads to less uniform densification and is not applicable to complex shapes. Therefore, in practice, the lubricant is most often admixed to the ferrous powders. The amount of lubricant is function of the application. Its content should be sufficient to minimize the friction forces at the die walls during the compaction and ejection of the parts. The amount of lubricant should, however, be kept as low as possible in the case of applications requiring high density level.
On the other hand, admixed lubricant most often reduces the strength of the green compact by forming a lubricant film between the metal particles which limits microwelding. When complex parts or parts with thin walls are to be produced, as well as when green parts have to be machined, parts with a high green strength are required. There is thus a need for a lubricant that would enable the manufacture of high green strength parts.
While cold compaction (at room temperature or rather 50-70.degree. C. in industrial conditions) is used most often, warm compaction (at temperatures up to 150.degree. C.-180.degree. C.) is also used when parts with high density are required. Indeed, warm compaction takes advantage of the fact that a moderate increase in the temperature of compaction lowers the yield strength of iron and steel particles and increases their malleability, leading to an increase of density for a given applied pressure.
However, the temperature used in warm compaction may affect the properties of the admixed lubricant and therefore affect the lubrication behavior during the compaction and ejection stages. Effectively, most of lubricants that are suitable for cold compaction cannot be used for warm compaction as this would cause increased die wear and produce parts with bad surface finish.
Conventional lubricants used in cold compaction include metallic stearates as zinc stearate or lithium stearate, or synthetic amide waxes as N,N'-ethylenebis(stearamide) or mixtures of metallic stearates and/or synthetic amide waxes. Polyethylene waxes, like CERACER 640, commercially available from Shamrock Technologies, have also been suggested as lubricants for cold compaction, but little literature exists on the use of this type of lubricant. Like synthetic amide waxes, polyethylene waxes have the advantage to decompose cleanly so that compacted parts are left free from residuals after the sintering operation. Shamrock Technologies report the use of their polyethylene wax lubricants to improve the green strength of metallic or ceramic bodies. On the other hand, Klemm et al., Adv.Powder Metall. & Particulate Matter., Vol. 2, 51-61 (1993), report in a study evaluating various P/M lubricants that the polyethylene wax tested lead to such a bad lubrication during the ejection of parts (high level of stick-slip), that they had to reject the idea to use this type of lubricant.
Accordingly, there is a need for an improved lubricant that would afford excellent lubrication in the course of both cold compaction and warm compaction of ferrous powders, and would enable the manufacture of parts having high green strength by cold compaction and having significantly higher density and green strength by warm compaction.